In the field of microwave radiation, it is well known that microwave furnaces are typically constructed with a fixed operating frequency. It has long been known that the interactions of various materials with microwaves are frequency dependent. These interactions may include curing rubber and sintering ceramics. It is therefore desirable to have a microwave furnace that can be operated over a broad frequency range.
Most microwave sources have a very narrow bandwidth because they employ a resonant cavity. Microwave ovens constructed for home use are provided with a magnetron which operates at 2.45 GigaHertz (GHz), which is an efficient frequency for heating water. Due to the coupling ability of a 2.45 GHz microwave to water, these ovens are used for cooking foods, drying, and other purposes wherein the principal material to be acted upon is water. However, it is well known that frequencies in this range are not optimal in all situations, such as with heating plasmas, sintering materials such as ceramics, and preparing films such as diamond films.
The use of frequency sweeping over a wide range as a means of mode stirring has important implications for the use of microwave power to sterilize medical equipment or contaminated wastes. In such uses it is crucial to eliminate "dead" areas in the cavity wherein sufficient power may not be received in order for complete sterilization. Electronic frequency sweeping may be performed at a high rate of speed, thereby creating a much more uniform time-averaged power density throughout the furnace cavity. The desired frequency sweeping may be accomplished through the use of a variety of microwave electron devices. A helix traveling wave tube (TWT), for example, allows the sweeping to cover a broad bandwidth (e.g., 2 to 8 GHz) compared to devices such as the voltage tunable magnetron (2.45+-0.05 GHz). Other devices have other characteristic bandwidths as will be disclosed hereinafter.
Further, fixed-frequency microwave ovens typically found in the home are known to have cold spots and hot spots. Such phenomena are attributed to the ratio of the wavelength to the size of the microwave cavity. With a relatively low frequency microwave introduced into a small cavity, standing waves occur and thus the microwave power does not uniformly fill all of the space within the cavity, and the unaffected regions are not heated. In the extreme case, the oven cavity is becomes practically a "single-mode" cavity.
Attempts have been made at mode stirring, or randomly deflecting the microwave "beam", in order to break up the standing modes and thereby fill the cavity with the microwave radiation. One such attempt is the addition of rotating fan blades at the beam entrance of the cavity.
Another method used to overcome the adverse effects of standing waves is to intentionally create a standing wave within a single-mode cavity such that the workpiece may be placed at the location determined to have the highest power (the hot spot). Thus, only the portion of the cavity in which the standing wave is most concentrated will be used.
It has been shown that the sintering behavior of various materials improves at higher frequencies, although the exact reasons are not known. However, existing state-of-the-art technology has made difficult the task of conducting a series of identical sintering experiments in which only the frequency is changed. This is due in large part to the fact that each microwave source is connected to a different furnace cavity. It is well known that the geometry of the furnace cavity is a parameter which must be considered in such experiments.
Furnaces incorporating gyrotron oscillators to produce fixed-frequency microwaves at 28 GHz have been reported in the literature. The gyrotron furnaces are capable of sintering some materials more efficiently than those ovens provided with the 2.45 GHz magnetrons. The gyrotron furnaces have specific applications in sintering materials such as ceramics. However, 28 GHz is not an efficient frequency with which to sinter all materials. It is desired to determine the most efficient frequencies to apply to a given material in a furnace with a cavity having a selected configuration.
The frequency for most efficient processing may vary for a given material as the heating process occurs. As a material changes phases, a varied frequency may be required. Thus, it may be desired to have the capability of varying the frequency in the heating process, allowing the tester to begin heating the specimen at one frequency and then change the frequency to maintain good coupling as the temperature rises. This may also be desirable when heating composite materials, where the varying materials efficiently react at different frequencies.
Other devices have been produced to change the parameters of the heating process of selected materials. Typical of the art are those devices disclosed in the following U.S. Patents:
______________________________________ U.S. Pat. No. Inventor(s) Issue Date ______________________________________ 3,611,135 D. L. Margerum Oct 5, 1971 4,144,468 G. Mourier Mar 13, 1979 4,196,332 A. MacKay B, et al. Apr 1, 1980 4,340,796 M. Yamaguchi, et al. Jul 20, 1982 4,415,789 T. Nobue, et al. Nov 15, 1983 4,504,718 H. Okatsuka, et al. Mar 12, 1985 4,593,167 O. K. Nilssen Jun 3, 1986 4,777,336 J. Asmussen Oct 11, 1988 4,825,028 P. H. Sinith Apr 25, 1989 4,843,202 P. H. Smith, et al. Jun 27, 1989 4,866,344 R. I. Ross, et al. Sep 12, 1989 4,939,331 B. Berggren, et al. Jul 3, 1990 ______________________________________
The subject matter disclosed by MacKay ('332) is further discussed in an article authored by MacKay B, et al., entitled "Frequency Agile Sources for Microwave Ovens", Journal of Microwave Power, 14(1), 1979. However, a microwave furnace having a wide frequency range has not been disclosed, except in the above-referenced co-pending application Ser. No. 07/792,103.
One barrier to the use of a wide frequency range is the difficulty of achieving efficient coupling of the microwaves to a multi-mode applicator cavity, particularly when such coupling means includes a dielectric window.
Therefore, it is an object of this invention to provide a microwave heating apparatus that can be operated over a wide frequency range.
Another object of the present invention is to provide a microwave heating apparatus which may be operated over a wide frequency range wherein the microwave source may be interchanged with microwave sources having other frequency ranges.
Still another object of the present invention is to provide a microwave heating apparatus whereby two or more microwave sources may be placed into the system in parallel.
A further object of the present invention is to provide a microwave heating apparatus including a plurality of microwave sources placed in parallel, wherein the microwave sources may be selectively and alternately used.
Still another object of the present invention is to provide such a microwave heating apparatus with multiple microwave sources whereby each microwave source is controlled by a common signal generator and power supply.
Yet another object of the present invention is to provide a microwave heating apparatus whereby frequency modulation may be used as a form of mode stirring to create a more uniform power distribution in a multi-mode cavity and the loads placed therein.